The present invention relates to instrumentation for rotating machinery. More particularly, the present invention relates to apparatus for monitoring and evaluating shaft vibration in terms of the ability of a bearing to withstand the vibrational motion.
A journal is that portion of a shaft which is supported by the bearings of the machinery. In a self-acting fluid film bearing, separation of the surfaces of the journal and bearing is effected by a lubricant film distributed between these surfaces by rotation of the journal. The resultant of all radial forces acting on the shaft is transmitted through this lubricant film and into the stationary structure of the apparatus through the bearings. If the resultant force exceeds the load-carrying ability of the bearing, metal-to-metal contact will occur at high relative surface speeds. This will result in catastrophic failure of the bearing and may destroy the associated apparatus.
A journal and bearing may experience both static and dynamic components of load during operation. The static component of load refers to the mean force over a large number of cycles of shaft revolution. The dynamic load refers to the rapid fluctuations of shaft radial force produced, for example, by an unbalance in the rotating shaft. The forces due to the dynamic component of load are superimposed upon the forces due to the static component of load. The time-varying dynamic forces produce alternating stresses in the bearing materials. Such alternating stresses are the primary cause of bearing metal fatigue which leads to loss of load-carrying ability in the bearing.
In the prior art, two methods have been employed to determine the severity of shaft vibrations. Seismic or accelerometer sensors have been employed casing-mounted on or near the bearings. In addition, conventional proximity probes have been employed either singly or in pairs to measure the relative motion between the bearing housing and the journal or its attached shaft.
The output of a seismic or accelerometer sensor is only indirectly related to the dynamic forces acting on the journal itself. That is, the bearing housing responds to the dynamic forces transmitted through the fluid film to the bearing housing as modified by the effective mass, stiffness and damping of the support structure. Thus, in order to determine the actual force on the journal, the result must be calibrated either through experience, test or theory to relate the level of housing vibration to the actual bearing load. Furthermore, such calibration may have to be performed on each different type of bearing housing and mounting arrangement due to the changes that these and other factors may have on the relationship between bearing housing motion and bearing load.
Measurements of shaft motion relative to the bearing housing using proximity sensors produce an ambiguous measure of bearing dynamic load. A given level of shaft vibration can correspond to a wide variety of dynamic loads. The relationship between shaft motion and dynamic load may depend on such factors as the type, size and geometry of the bearing together with its operating conditions of speed, static load and lubricant viscosity. Thus, for a given level of shaft vibration, an acceptable bearing load may be produced under certain operating conditions and an unacceptable load may be produced under another set of operating conditions.
In addition, proximity sensors as used in the prior art have produced signals related to the motion of the shaft along one axis. If this axis is not aligned with the axis of maximum displacement of the shaft, an imperfect measurement of shaft vibrational motion is produced. Shaft vibrational motion follows an elliptical path having a major and a minor axis. Thus, an arbitrarily located proximity sensor is unlikely to be aligned with the major axis and to thereby sense maximum vibratory motion. An an added complication, the axes of the ellipse may rotate under changes in conditions of shaft speed and static load. In this case, it is not possible to position a proximity sensor along an axis which is aligned with the major axis of the ellipse under all conditions.
Two proximity sensors are often employed disposed with their sensing axes 90 degrees to each other. However, in this typical arrangement, each proximity sensor still produces readings which are related only to the shaft motion along its axis and, except for temporary fortuitous orientation of the elliptical axes of shaft motion, fail to produce signals related to maximum displacement of the journal in the bearing.
The American Petroleum Institute publishes standards describing proximity probe installation requirements (API 670), allowable shaft vibration for mechanical equipment such as steam turbines (API 612), gas turbines (API 616) and gears (API 613). The information in these standards are included herein by reference.
The individual signals from a pair of proximity probes are sometimes combined by displaying them together on an instrument such as an oscilloscope. This provides a visual display of the elliptical shaft orbit for a pair of proximity probes oriented 90 degrees to each other. With such an instrument arrangement, the maximum vibratory displacement or major axis of the elliptical shaft orbit may be measured.
Even when maximum vibratory displacement is determined, however, only a partial answer to the possibility of bearing destruction is obtained. For a given bearing type and size, the magnitude of bearing dynamic force depends upon both the maximum vibratory displacement, the orientation of the elliptical orbit and the mean position of the journal relative to the bearing.
The mean position of the journal is determined by the static component of journal load. The mean journal position also establishes the dynamic bearing coefficients. These coefficients, together with the shaft vibration parameters defining the shaft orbital motion, establish the maximum force on the bearing. Conventional monitoring systems employing proximity sensors do not consider these factors and therefore are incapable of providing a measure of dynamic bearing load.